1. Field of the Invention
The present invention relates to a process for producing an extrusion laminated film and an extrusion laminated sheet. More particularly, the present invention relates to a process for producing an extrusion laminated film and an extrusion laminated sheet with at least one resin for extrusion laminating selected from the group consisting of polyethylene resins, polypropylene resins, ethylene-vinylester copolymer resins and ethylene-(metha)acrylate copolymer resins, which enables production of an extrusion laminated film and an extrusion laminated sheet comprising a plastic substrate having films firmly adhered thereto without using any chemical primer agent.
2. Description of the Related Art
Production of extrusion laminated films and sheets comprising stacked different film materials such as plastic, paper, metal foils and the like having additionally other properties than those possessed by single material, for example, increased strength, gas impermeability, moisture-proofing property, heat-sealing property, improved appearance, and the like has been widely conducted and the products have primarily been used in a variety of wrappings. Such laminated films and sheets have been produced by any one of methods such as dry laminating, wet laminating, hot laminating, extrusion laminating, coextrusion laminating, and the like, selected depending upon characteristics to be utilized. Wrapping materials comprising a substrate having a heat-seal layer have been produced by the popularly used extrusion laminating method to form the heat-seal layer on the substrate which is advantageous in cost. Materials to be generally used for the heat-seal layer include polyolefin resins such as polyethylenes, polypropylenes, and ethylene copolymers, and ionomers. However, polyolefin resins have been used in large qualities from the economical standpoint.
These resins are generally melt-extruded and laminated onto the adhesive surface of a substrate which has been precoated with a chemical primer to promote the adhesion between the substrate and the resins. The chemical primers to be used include organic titanate, organic isocyanate, and polyethyleneimine type adhesives. When used, these adhesives are usually diluted with an organic solvent such as toluene, ethyl acetate, methanol, hexane and the like. However, these methods using the chemical primers produce problems such as increased production costs owing to the use of expensive chemical primers, required complicated steps of applying and drying the chemical primers, scattering of organic solvents harmful to human beings into the air when the chemical primers are dried by evaporating the organic solvents therein causing environmental hazards in and around workplaces, occurrence of fire due to the use of inflammable organic solvents, and limited utilization of the end film or sheet products in food wrappings owing to the odors attributable to the residual organic solvents in the chemical primers on the products.
As a method without using the chemical primers, an attempt has been made to produce a laminate by melt-kneading an ethylene copolymer produced by copolymerization of (a) ethylene, (b) unsaturated polybasic acid, and (c) an unsaturated monomer selected from lower alkyl acrylate esters, lower alkyl methacrylate esters and vinyl esters, extruding said molten copolymer at a temperature of 150.degree. C. to 330.degree. C. into a molten web, treating the molten web with ozone and laminating the molten web onto a substrate under pressure with the ozone treated surface of the molten web being adhered onto the surface of the substrate as described in Japanese Patent KOKAI (Laid-open) No. Hei 4-368845, wherein however only biaxially stretched nylon film was practically employed, and no practical use of ethylene-vinyl alcohol copolymers as substrate was disclosed. In addition, the methods using the ethylene copolymers containing the unsaturated polybasic acids as comonomer component and having adhesive function suffer from added production cost, lower release property in the extrusion laminating process of laminates with an increase in the amount of low melting components to limit processing temperatures as well as cumbersome exchange of resins in the extruder which may be required by the methods.
Alternatively, a method for producing an extrusion laminated film by treating the surfaces of an ethylene-.alpha.-olefin copolymer molten web to be adhered onto a substrate with ozone and laminating the molten web onto the substrate without applying any chemical primer to the substrate, or a method for producing an extrusion laminated film by coextruding a modified polyolefin resin grafted with unsaturated carboxylic acids having an adhesive property onto a substrate without any chemical primer through a combination with a coextruder have been known, as disclosed in CONVERTIC (8) p. 36, (1991). However, the laminates obtained by these methods have disadvantages of insufficient adhesion strength between the laminated resin layer and the substrate limiting the area where the laminates are applied. Moreover, the method using the adhesive modified polyolefin resins grafted with unsaturated carboxylic acids requires disadvantageously the use of coextruder with an increase in production cost as well as cumbersome exchange of resins in the extruder.